@ LostInTime
I've re-read the passage in Changes chapter 30:
“You know my price, mortal. Speak it.” “You want me to become the Winter Knight,” I whispered. A laugh, both merry and cold, bubbled beneath her response. “Yes.” “I will,” I said. “With a condition.” “Speak it.” “That before my service begins, you restore my body to health. That you grant me time enough to rescue my daughter and take her to safety, and strength and knowledge enough to succeed. And you give me your word that you will never command me to lift my hand against those I love.”
Harry just told Mab those were the conditions. She never actually
agreed to anything he said. Neither through words nor through gestures or eye-blinking.
Meaning she can tell him to do anything she wants.
And therefore has no need to use the last favour to order him to kill anyone. She can just command him as her knight.
But I agree, there is the last favour left open. She only said in the previous books, that she would consider the favours done, if hr agreed to become the Winter Knight instead of doing the favour.
As he became the Knight for his own reasons, there was no favour deal involved. Just the time, power and knowledge to save his child and the restoration of his body. And she did not even agree to those either. But she granted part of his requests. So imho she fooled him into thinking she granted all requests.
As long as Harry remains Winter Knight, Mab has no need to call in the last favour. But once he gets out of her service (and I do think, he will figure out how to do so without dying, someday), there will be the last favour for her to call in.
Or she could sell it to somebody else like Lea sold it to her.
